1. Field of the Invention
The present invention relates generally to methods and devices for short term inspection of structures, or long term monitoring of the health of a structure. The present invention relates more specifically to a magnetostrictive sensor based system for short term inspection or long term monitoring of the health of a structure.
2. Description of the Related Art
Magnetostrictive effect refers to the phenomena of a physical dimension change in ferromagnetic materials that occurs through variations in magnetization. In magnetostrictive applications, the generation and detection of mechanical waves is typically achieved by introducing a pulse current into a transmitting coil adjacent to a ferromagnetic material. The change in magnetization within the material located near the transmitting coil causes the material to change its length locally in a direction parallel to the applied field. This abrupt local dimension change, which is the magnetostrictive effect, generates a mechanical wave (called guided wave) that travels through the ferromagnetic material with a certain fixed speed (which is usually less than the speed of sound). When the mechanical wave is reflected back from the end of the ferromagnetic material, or from a defect in the ferromagnetic material, and reaches a detection coil, the mechanical wave generates a changing magnetic flux in the detection coil as a result of the inversed magnetostrictive effect. This changing magnetic flux induces an electric voltage within the detection coil that is proportional to the magnitude of the mechanical wave. The transmitting coil and the detection coil can be identical.
Advantages of using the magnetostrictive effect in nondestructive evaluation (NDE) applications include (a) the sensitivity of the magnetostrictive sensors, (b) durability of the magnetostrictive sensors, (c) no need to couple the sensor to the material being investigated, (d) long range of the mechanical waves in the material under investigation, (e) ease of implementation, and (f) low cost of implementation.
The use of magnetostrictive sensors (MsS) in the nondestructive evaluation (NDE) of materials has proven to be very effective in characterizing defects, inclusions, and corrosion within various types of ferromagnetic and non-ferromagnetic structures. A MsS launches a short duration (or a pulse) of guided waves in the structure under investigation and detects guided wave signals reflected from anomalies such as defects in the structure. Since guided waves can propagate long distances (typically 100 feet or more), the MsS technique can inspect a global area of a structure very quickly. In comparison, other conventional NDE techniques such as ultrasonics and eddy current inspect only the local area immediately adjacent to the probes used. Therefore, the use of magnetostrictive sensors offers a very cost effective means for inspecting large areas of steel structures such as strands, cables, pipes, and tubes quickly with minimum support requirements such as surface preparation, scaffolding, and insulation removal. The ability to use magnetostrictive sensors with little preparation of the object under inspection derives from the fact that direct physical contact between the sensors and the material is not required.
Efforts have been made in the past to utilize magnetostrictive sensor technologies in association with the inspection of both ferromagnetic and non-ferromagnetic materials. Included in these efforts are systems described in U.S. Pat. Nos. 5,456,113; 5,457,994; and 5,501,037, which are each commonly owned by the assignee of the present invention. The disclosures of U.S. Pat. Nos. 5,456,113; 5,457,994; and 5,501,037, provide background on the magnetostrictive effect and its use in NDE and are therefore incorporated herein by reference. These efforts in the past have focused primarily on the inspection of pipe, tubing and steel strands/cables wherein the geometry of the structure is such that the cross-sectional diameter is small in comparison to the length of the structure. While these systems and their application to longitudinal structures find significant applications, there are yet other structures that could benefit from the use of magnetostrictive based NDE.
Other efforts have been made in the past to utilize sensors that measure magnetic flux and/or acoustic waves in structural materials. These efforts have included those described in the following patents:
U.S. Pat. No. 3,555,887 issued to Wood on Jan. 19, 1971 entitled Apparatus for Electroacoustically Inspecting Tubular Members for Anomalies Using the Magnetostrictive Effect and for Measuring Wall Thickness. This patent describes a system designed to direct a mechanical wave through the thickness dimension of a long tubular member. The sensitivity of the device is limited to the directing of a wavefront normal to the surface of the material under inspection and immediately back to a sensor when reflected from an opposite wall or an anomaly.
U.S. Pat. No. 4,881,031 issued to Pfisterer, et al. on Nov. 14, 1989 entitled Eddy Current Method and Apparatus for Determining Structure Defects in a Metal Object Without Removing Surface Films or Coatings. This patent describes a method for establishing localized eddy currents within ferromagnetic materials and recognizes the presence and effect of a coating in order to identify and quantify corrosion beneath the coating. As with other eddy current methods, the ability to inspect a material is limited to the area immediately adjacent to the sensor.
U.S. Pat. No. 5,544,207 issued to Ara, et al. on Aug. 6, 1996 entitled Apparatus for Measuring the Thickness of the Overlay Clad in a Pressure Vessel of a Nuclear Reactor. This patent describes a system directed solely to the measurement of magnetic field variations that result from the distribution of the magnetic field through overlays of varying thickness. The system utilizes a magnetic yoke that is placed in close contact with the surface of the overlay clad of the pressure vessel.
U.S. Pat. No. 5,687,204 issued to Ara, et al. on Nov. 11, 1997 entitled Method of and Apparatus for Checking the Degradation of a Pressure Vessel of a Nuclear Reactor. This patent describes a system similar to the earlier issued Ara, et al. patent and utilizes a magnetic yoke having an excitation coil and a magnetic flux measuring coil that are placed in close contact with the inner wall of the pressure vessel. The hysteresis magnetization characteristics formed by the magnetic yoke and the pressure vessel wall are measured. Degradation of the material comprising the pressure vessel is inferred from a determination of the hardness of the material which is determined from the coercive forces obtained by analyzing the hysteresis characteristics of the magnetization.
In general, a magnetostrictive sensor consists of a conductive coil and a means for providing a DC bias magnetic field in the structure under inspection. The means for providing a bias magnetic field can include the use of either permanent magnets or electromagnets. In a transmitting magnetostrictive sensor, an AC electric current pulse is applied to the coil. The resulting AC magnetic field (a changing magnetic field) produces guided waves in an adjacent ferromagnetic material through the magnetostrictive effect. For pipes, cables, tubes, and the like, the waves are typically launched along the length of the longitudinal structure. In the receiving magnetostrictive sensor, a responsive electric voltage signal is produced in the conductive coil when the guided waves (transmitted or reflected from anomalies within the material) pass the sensor location, through the inverse magnetostrictive effect.
With MsS techniques, defects are typically detected by using the pulse-echo method well known in the field of ultrasonics. Since the sensor relies on the magnetostrictive behavior found in ferrogmagnetic materials, this technology is primarily applicable to the inspection of ferromagnetic components such as carbon steel piping or steel strands. It is also applicable, however, to the inspection of nonferrous components if a thin layer of ferromagnetic material, such as nickel, is plated or coupled onto the component in the area adjacent to the magnetostrictive sensors.
The magnetostrictive sensor technique has the advantage of being able to inspect a large area of material from a single sensor location. Such sensors have, for example, been used to accurately inspect a length of pipe or cable of significantly more than 100 feet. Further, magnetostrictive sensor techniques are comprehensive in their inspection in that the methods can detect both internal and external defects, thereby providing a 100% volumetric inspection. The techniques are also quite sensitive, being capable of detecting a defect with a cross-section less than 1% of the total metallic cross-section of cylindrical structures such as pipes, tubes, or rods. Finally, as indicated above, magnetostrictive sensor techniques do not require direct physical contact between the component surface and the sensor itself. This eliminates the need for surface preparation or the use of a couplant.
In recent years, there have been many reported occurrences of steel containment liners degrading at commercial nuclear power plants. Due to the aging of such facilities and the increased requirements for inspection, incidents of degradation are likely to increase. The structural degradation of these liners, especially corrosion damage, is an important concern since the liners are designed to provide a leak-tight pressure boundary for the nuclear containment. Many other industrial uses of plate type ferromagnetic materials could benefit from more frequent inspections to determine the state of deterioration, the location of faults, and the likelihood of failure. In most instances in the past, inspections of large plate type objects (such as large aboveground storage tanks) have required either very expensive off-line inspections or statistical samplings of randomly selected local areas that are for the most part less than reliable. It has heretofore been difficult to carry out a thorough inspection of a plate type structure, or a structure comprised of a plurality of plate type sheets of material, without high cost and long down time for the object under inspection. It would be desirable to use the magnetostrictive sensor technique for detecting and locating various anomaly characteristics within plate type materials. Such techniques could be used for detecting and locating wall thickness reductions in liners, such as those described above, that might be caused by corrosion over time. If such a system were applicable, it would be possible to inspect otherwise inaccessible regions of containment liners and the like that are either imbedded in concrete or adjacent to flooring or equipment that cannot be moved.
It would therefore be desirable to implement magnetostrictive sensor techniques in conjunction with plate type structures in a manner similar to, and with the accuracy of, such systems utilized in conjunction with cylindrical structures. It would be desirable if an inspection of plate type and cylindrical structures could be carried out in an efficient manner that did not require full access to the surface of the plate or the inner or outer surface of cylindrical structures such as pipes and tubes. Such a magnetostrictive sensor system would be able to investigate large volumes of a plate type or cylindrical structure, including pipes and tubes, and would provide a cost effective global inspection of the structure.
It is therefore an object of the present invention to provide a sensor device for implementing magnetostrictive based NDE in association with pipes and tubes in order to evaluate the condition of the structures and to determine the presence of anomalies indicative of fractures, deteriorations, and the like.
It is a further object of the present invention to provide a magnetostrictive sensor appropriate for use in conjunction with the inspection of pipes and tubes that is capable of transmitting and receiving guided waves within the pipes and tubes and generating signals representative of the characteristics of such waves appropriate for the analysis and detection of anomalies therein.
It is a further object of the present invention to provide magnetostrictive sensor devices appropriate for use in conjunction with the inspection of pipes and tubes that inspect the entire structure for anomalies, corrosion, fractures, and the like in a cost effective manner.
It is a further object of the present invention to provide a method for the inspection of pipes and tubes that includes the use of a magnetostrictive sensor specifically adapted for directing guided waves along the length of the pipe or tube and detecting such waves as may be reflected from anomalies along the pipe or tube.
It is yet another object of the present invention to provide a method and apparatus for nondestructive evaluation of pipes and tubes utilizing magnetostrictive sensors that generate and detect shear horizontal waves along the length of the item being inspected.
It is yet another object of the present invention to provide a magnetostrictive sensor that is suitable for low frequency operation (200 kHz or less), has good sensitivity and long inspection range, and is relatively tolerate to liftoff.
It is still another object of the present invention to provide a method and apparatus for nondestructive evaluation of pipes using magnetostrictive sensors that propagate guided waves in a circumferential direction around the pipe.
Another object of the present invention is to provide a method and apparatus for nondestructive evaluation of pipes and tubes using magnetostrictive sensors with torsional waves that has better defect detectability particularly in liquid filled pipes or tubes.
Still another object of the present invention is to provide a method and apparatus for the nondestructive evaluation of pipes and tubes that requires no permanent DC bias magnets or electromagnets and, thus is easier to apply.
Another object of the present invention is to provide a method and apparatus for the nondestructive evaluation of pipes and tubes that has a reduced setup time and therefore a lower inspection cost.
In fulfillment of these and other objectives, the present invention provides a method and apparatus for implementing magnetostrictive sensor techniques for the nondestructive evaluation of plate type structures such as walls, vessels, enclosures, and the like. The system includes magnetostrictive sensors specifically designed for application in conjunction with welded plate type structures that generate guided waves in the plates which travel through the plate in a direction parallel to the surface of the plate. Similarly structured sensors are positioned to detect the guided waves (both incident and reflected) and generate signals representative of the characteristics of the guided waves detected. The system anticipates the use of either discrete magnetostrictive transmitters and receivers or the use of a single magnetostrictive sensor that operates to both transmit and detect the guided waves. The sensor structure is longitudinal in nature and generates a guided wave having a wavefront parallel to the longitudinal direction of the sensor. Appropriate electronics associated with the process of generating the guided waves and controlling the propagation direction of the generated wave through the magnetostrictive transmitter as well as detecting, filtering, and amplifying the guided waves at the magnetostrictive receiver, are implemented as is well known in the art. Signal analysis techniques, also known in the art, are utilized to identify anomalies within the plate type structure. The method utilizes pattern recognition techniques as well as comparisons between signal signatures gathered over time from the installation of the structure under investigation to a later point after deterioration and degradation may have occurred.
The magnetostrictive sensors can also be used to detect defects in cylindrical structures such as to detect defects in electric resistance welding, such as in pipes that are welded along a seam thereof. For example, a magnetostrictive transmitter can be placed on one side of the pipe being investigated and a magnetostrictive receiver on the other side of the pipe. By propagating a guided wave in circumferential direction around the pipe, any defects in the pipe can immediately be detected, such as in the area of the weld.
For generation and detection of the symmetrical (S) or the anti-symmetrical (A) Lamb wave mode in a plate type structure, the DC magnet or field required for MsS operation is applied parallel to the direction of wave propagation. For generation and detection of the shear horizontal (SH) wave mode, the DC magnetic field required for MsS operation is applied perpendicular to the direction of wave propagation. Due to the enclosed nature of cylindrical structures such as pipes and tubes, the shear horizontal wave can be induced to act as a torsional wave along the length of the pipe or tube. The generation of a shear horizontal or torsional wave along the length of the pipe or tube allows defect detectability that will not be hampered by the presence of liquid in the pipe or tube.
Current flow along the longitudinal axis of a pipe or tube will cause magnetization of a ferromagnetic pipe or tube in the circumferential direction. This magnetization can be used for the transmission and detection of torsional waves that flow along the pipe and tube and any reflections thereof. The reflections may be from anomalies or defects in the pipe or tube.
Also, a thin ferromagnetic strip that is magnetized in the circumferential direction may be wrapped around and held tightly against the pipe or tube. Thereafter, a torsional wave may be generated or detected where the ferromagnetic strips are located along the pipe or tube. The circumferential magnetization around the pipe or tube is in the ferromagnetic strip. It is very important to hold the ferromagnetic strip in tight surface contact with the pipe or tube so that the full effect of the torsional wave can be felt and detected in either the transmitter or receiver coils adjacent thereto.
In another embodiment of the present invention, a thin strip of a ferromagnetic material that can retain residual magnetization, such as nickel, is prepared an appropriate width and length. The width of the strip depends upon the operating frequency of the magnetostrictive device. The length of the strip depends upon the structure to be monitored. For example, for a pipe, the length is slightly shorter than the circumference of the pipe. For a plate-type structure, the length is typically 10 inches or less.
Residual magnetization is induced along the length of the strip by applying an external magnetic field to the strip along its length and then removing the external magnetic field. Afterwards, the strip is coupled to the structure to be monitored with an appropriate material, such as epoxy. For a pipe, the strip is bonded around the circumference of the pipe. For a plate-type structure, the strip is bonded normal to the direction of wave propagation to be used for inspection. For short term inspection, a viscous couplant, such as honey, may be used to couple the strip on a temporary basis to the structure being inspected.
After coupling the strip to the structure to be inspected, a coil is either wrapped around or placed adjacent to the magnetized strip. A minimum of two strips and coils are used, one for transmitting and one for receiving the magnetostrictive signals.
For long term monitoring, the transmitters and receivers are encased or covered in a manner to protect them from the environment. Wires from the magnetostrictive transmitters and receivers are easily accessible whereby the transmitters and sensors can be electrically monitored by appropriate magnetostrictive instrumentation.
For short term inspection, the signal obtained indicates if there is a defect in the structure being inspected. For long term monitoring, a baseline signal is obtained and stored in the computer. Thereafter, additional signals are obtained periodically from the magnetostrictive transmitters and receivers with changes in the signal indicating changes in the structure being inspected.
The guided wave normally used in the method just described for piping applications is a torsional wave and in plate-type structures is a shear horizontal wave.